Present equipment that use precision sieves are batch type units where the powder is processed on reinforced screen and collected directly on the screen. This requires that for each batch that is processed the mounted screens have to be handled at the beginning and end of the process, resulting in the possible damage to the screen. Examples of this type of equipment include the Alpine Air-Jet Sieve and the ATM Sonic Sifter.
The concept of passing particles through an electrostatic field for the purpose of propelling the particles beyond a screen is disclosed in my U.S. Pat. No. 3,635,340. This patent discloses the use of particle momentum produced by pulling the particles across a field to propel particles through a printing screen. Further, it is also disclosed that this propulsion of the particles beyond a second electrode may be used for possible particle classification. This patent, however, does not recognize or utilize particle oscillation as the vehicle for screen trials. It was further disclosed that particle separation could be accomplished by passing the particles across a horizontal conveying electrode which relied upon vibration to move the particles to the screen or stencil electrode mounted above the horizontal vibrating electrode.
Another example of the use of electrostatic separation of particles known in the prior art is shown in U.S. Pat No. 2,361,946 to Johnson et al. The Johnson et al patent discloses an electrostatic separation of particles which utilizes direct fields or alternating fields for the production of particle dispersion, agitation and propulsion between electrodes. The Johnson et al patent utilizes an inclined electrode configuration where the sieve electrode is placed below an upper electrode. Where it is desired to use direct potentials, the upper electrode is a bare, solid, metallic electrode. A solid upper electrode has been found to be required in apparatus which use an inclined electrode configuration. The use of such electrodes, however, allows fine particles to adhere to the surface in local areas and thus produces variations in the electrical field strength, and sparking and possible stoppage of the process.
In the Johnson et al patent, the principal phenomena relied upon is the attraction and repulsion of particles between electrodes of an opposite charge. A particle by reason of the charge received from the lower sieving electrode is propelled upwardly to the upper electrode plate from which, by contact therewith, it receives the opposite charge and is propelled back down to the lower electrode. Particles which do not actually touch the upper electrode may also be propelled downward by gravity.
In the Johnson et al patent, there is no recognition of the potential use of the inherent oscillation of a dispersed group of particles between electrodes of a like charge.
The prior art has also utilized electrostatic fields for the separation of particles through the technique of passing the particle through a field and relying upon the mass-to-charge ratio to accomplish the separation. An example of this is found in U.S. Pat. No. 2,803,344 to Morrison, which utilizes gravity to separate the particles as they pass across an electrostatic field. This technique, however, does not rely upon the oscillating motion produced by the electrostatic dispersion to propel the particles to a classifying screen. In this apparatus, there is no requirement that the particles oscillate during separation since there is no classification screen against which trails are made.
Another patent which uses electrostatic separation, but which does not utilize the oscillation of particles in free fall against a sieve under the influence of an electrostatic field, is Brastad, el. al, U.S. Pat. No. 2,848,108. Brasted specifically rejects the electrostatic dispersion and transport of particles in suspension, used by the present invention, in favor of mechanical vibration of electrodes to transport flour resting on the electrodes. Brasted uses a solid lower electrode 20, which is essentially horizontal (inclined no more than .+-.71/2.degree.)--in fact Brasted states that an inclination of over 15.degree. is fatal. Flour is deposited upon, and supported by, the lower solid electrode, which is mechanically vibrated. This vibration of the lower solid electrode is the medium by which the powder (flour) is transported through the apparatus. The flour is sorted by the differential attraction of some particles to the upper electrode 22. Large openings 94 (or slots 154) may be provided in the upper electrode, separated by flat unperforated areas 96 and with raised rims 98, but the upper electrode does not serve as a sieve--the particles of flour are attracted upward and pass through the openings (dependent entirely upon the electrostatic attraction and not upon the hole size as in a sieve) and then rest upon the upper surface of the electrode. The upper electrode is then vibrated to transport the flour resting upon it. The side panels 100 of the upper electrode extend upward (away from the other electrode) and serve only to keep powder resting on the upper electrode from falling off the edges--they cannot have any effect upon the strength of the field.
My previous patent, U.S. Pat. No. 4,172,028 (1979), utilized two vertical screens opposing one another. At that time the emphasis was on separating fine particles, less than 44 microns (325 mesh). Processing on both sides proved to be effective for low specific gravity materials, &lt;5.00 g/ml, but for larger particles with higher specific gravities a single screen is more efficient when operating at lower angles, 10 to 40 degrees from the horizontal, FIG. 1.
Another of my earlier patents, U.S. Pat. No. 4,071,169 (1978), suggested the use of angularly adjustable electrodes used for the purpose of sieving powders. This equipment had several flaws, one of which was in the powder input area (52) in FIG. 5. When the equipment was in zero to ten degrees operating position powders flowed in both directions--backwards and away from the conveying direction--resulting in the loss of powder.
Another problem developed with the converging edges of upper or lower electrodes, in figure eight. The converging edges of these electrodes do confine the powder to the processing area, but with a reduction of the electric field in the center, or major processing area. The end result is lower particle velocity and number of trials for sieving efficiently.
Furthermore, each of these devices required manual removal of the collection pans for the fines (sieved material) or the coarse material. This labor was increased by the problems associated with the dispersion of the materials both laterally and lengthwise (especially in nearly horizontal operation) across the electrodes.